Now let’s say that our chat application also has a contact list, and we want to render names of online users with a green color. We could copy and paste similar logic above into our FriendListItem component but it wouldn’t be ideal:

Instead, we’d like to share this logic between FriendStatus and FriendListItem.

Traditionally in React, we’ve had two popular ways to share stateful logic between components: render props and higher-order components. We will now look at how Hooks solve many of the same problems without forcing you to add more components to the tree.

Extracting a Custom Hook

When we want to share logic between two JavaScript functions, we extract it to a third function. Both components and Hooks are functions, so this works for them too!

A custom Hook is a JavaScript function whose name starts with ”use” and that may call other Hooks. For example, useFriendStatus below is our first custom Hook:

There’s nothing new inside of it — the logic is copied from the components above. Just like in a component, make sure to only call other Hooks unconditionally at the top level of your custom Hook.

Unlike a React component, a custom Hook doesn’t need to have a specific signature. We can decide what it takes as arguments, and what, if anything, it should return. In other words, it’s just like a normal function. Its name should always start with use so that you can tell at a glance that the rules of Hooks apply to it.

The purpose of our useFriendStatus Hook is to subscribe us to a friend’s status. This is why it takes friendID as an argument, and returns whether this friend is online:

Is this code equivalent to the original examples? Yes, it works in exactly the same way. If you look closely, you’ll notice we didn’t make any changes to the behavior. All we did was to extract some common code between two functions into a separate function. Custom Hooks are a convention that naturally follows from the design of Hooks, rather than a React feature.

Do I have to name my custom Hooks starting with “use”? Please do. This convention is very important. Without it, we wouldn’t be able to automatically check for violations of rules of Hooks because we couldn’t tell if a certain function contains calls to Hooks inside of it.

Do two components using the same Hook share state? No. Custom Hooks are a mechanism to reuse stateful logic (such as setting up a subscription and remembering the current value), but every time you use a custom Hook, all state and effects inside of it are fully isolated.

How does a custom Hook get isolated state? Each call to a Hook gets isolated state. Because we call useFriendStatus directly, from React’s point of view our component just calls useState and useEffect. And as we learnedearlier, we can call useState and useEffect many times in one component, and they will be completely independent.

Tip: Pass Information Between Hooks

Since Hooks are functions, we can pass information between them.

To illustrate this, we’ll use another component from our hypothetical chat example. This is a chat message recipient picker that displays whether the currently selected friend is online:

This lets us know whether the currently selected friend is online. If we pick a different friend and update the recipientID state variable, our useFriendStatus Hook will unsubscribe from the previously selected friend, and subscribe to the status of the newly selected one.

useYourImagination()

Custom Hooks offer the flexibility of sharing logic that wasn’t possible in React components before. You can write custom Hooks that cover a wide range of use cases like form handling, animation, declarative subscriptions, timers, and probably many more we haven’t considered. What’s more, you can build Hooks that are just as easy to use as React’s built-in features.

Try to resist adding abstraction too early. Now that function components can do more, it’s likely that the average function component in your codebase will become longer. This is normal — don’t feel like you have to immediately split it into Hooks. But we also encourage you to start spotting cases where a custom Hook could hide complex logic behind a simple interface, or help untangle a messy component.

For example, maybe you have a complex component that contains a lot of local state that is managed in an ad-hoc way. useState doesn’t make centralizing the update logic any easier so might you prefer to write it as a Redux reducer:

Reducers are very convenient to test in isolation, and scale to express complex update logic. You can further break them apart into smaller reducers if necessary. However, you might also enjoy the benefits of using React local state, or might not want to install another library.

So what if we could write a useReducer Hook that lets us manage the local state of our component with a reducer? A simplified version of it might look like this:

The need to manage local state with a reducer in a complex component is common enough that we’ve built the useReducer Hook right into React. You’ll find it together with other built-in Hooks in the Hooks API reference.